1. Field of the Invention
The present invention relates to a thread cutting knife device disposed under a bed of a sewing machine.
2. Description of the Prior Art
A thread cutting knife device known in the prior art is disposed in a thread cutting unit of a sewing machine. The thread cutting unit is used for cutting a stitching thread and consists of a needle thread looping portion at the side of a cloth and a bobbin thread which is located under the sewn cloth, namely, between a throat plate on which a sewn cloth is placed and a hook. The hook cooperates with a needle as it moves from the surface of the sewn cloth to an upper stop position raised above the surface of the sewn cloth. The sewing machine provided with this type of unit is called a thread cutting sewing machine.
When prior art thread cutting units are used, it is desirable to cut off the needle thread looping portion and the bobbin thread under a portion as close as possible to the throat plate while keeping the needle thread and the bobbin thread held properly on the side of the sewing machine for forming an appropriate stitch from the first stitch at the next sewing operation after completion of sewing operation. When the cutting of the needle thread and the bobbin thread takes place at the portion as close as possible to the throat plate, thread ends remaining on the needle thread and the bobbin thread will be shortened so that it is not necessary to employ an additional process to remove the thread ends in order to improve the working efficiency. However, the thread ends of the needle thread and the bobbin thread remaining on the sewn cloth after completion of the sewing operation must be cut off by a sharp blade to insure the quality of the sewn cloth.
However, it is difficult to separate the needle thread looping portion at the side of the cloth and the bobbin thread from the needle thread looping portion at the side of the needle in order to cut off the needle thread and the bobbin thread at the ideal position close to the sewn cloth. Furthermore, the blade of the thread cutting knife and the needle thread and the bobbin thread are brought into extremely close contact with each other whereby long ends of the needle thread, and the bobbin thread remain on the sewn cloth. Consequently, the sewn cloth can be damaged by the blade of the thread cutting knife whereby the appearance of the sewn cloth deteriorates.
For example, the thread cutting unit of the sewing machine disclosed in Japanese Patent Publication No. 57-54158 comprises a thread cutting knife movable along an outer periphery of a hook and a loop spreading plate. The thread cutting knife has a flat plate shape and is disposed over the loop spreading plate having a flat plate shape so that the needle thread looping portion at the side of the cloth and the bobbin thread are separated from the needle thread looping portion at the side of the needle by a forked catch portion and caught thereby, whereby the blade at the tip end of the thread cutting knife crosses the catch portion at the tip of the loop spreading plate to thereby cut off the stitching thread in a tension state. As a result, the portion cut off by the blade is not stable and the needle thread and the bobbin thread can be damaged by the blade. Furthermore, the separation of the loop spreading plate at the side of the cloth and the bobbin thread from the loop spreading plate at the side of the needle is incompatible with the reduction of length of thread ends remaining on the loop spreading plate at the side of the cloth and the bobbin thread.